WO2020040719A1

WO2020040719A1 - Interlock between materials in multiple material additive manufacturing

Info

Publication number: WO2020040719A1
Application number: PCT/US2018/047017
Authority: WO
Inventors: Mark R. BURHOP
Original assignee: Siemens Aktiengesellschaft; Siemens Corporation
Priority date: 2018-08-20
Filing date: 2018-08-20
Publication date: 2020-02-27

Abstract

For forming (74) an interlock between materials (14, 15) in multiple material additive manufacturing, different materials (14, 15) are three-dimensionally (3D) printed together as a unified object (20). Rather than relying on poor bonding between the materials (14, 15) alone, the contract surface (26) between the materials (14, 15) is shaped to interlock. Translation and/or rotation along any degree or degrees of freedom is limited by the interlocking shape of the contract surface (26). Any bonding of the printed materials (14, 15) and the shape of the contract surface (26) hold the portions (22, 24) of the unified object (20) together. The need to bind together separately formed pieces as a post processing step is avoided.

Description

INTERLOCK BETWEEN MATERIALS IN MULTIPLE MATERIAL ADDITIVE

MANUFACTURING

BACKGROUND

[0001] The present embodiments relate to additive manufacturing. Some forms of additive manufacturing machines print with multiple materials. For example, Fused Deposition Machines (FDM) have dual nozzles for extruding two different molten plastics. By alternating between nozzles, some regions of a single object can be printed with one nozzle and one material while other regions are printed by the second nozzle and other material. To specify the regions, a design is usually cut to create different region pieces with each region assigned a different material. The two materials may not bond strongly to each other during the printing process, creating inferior mechanical properties and/or allowing delamination.

[0002] Separate parts may be printed independently and connected to form the object. Fasteners, glues, welding, and other joinery are used to connect multiple objects in an assembly as a post process after printing the separate objects. This extra post process is expensive and adds difficulty in manufacture of the object.

SUMMARY

[0003] By way of introduction, the preferred embodiments described below include methods, systems, instructions, and computer readable media for forming an interlock between materials in multiple material additive

manufacturing. Different materials are three-dimensionally (3D) printed together as a unified object. Rather than relying on poor bonding between the materials alone, the contact surface between the materials is shaped to interlock. Translation and/or rotation along any degree or degrees of freedom is limited by the interlocking shape of the contact surface. Any bonding of the printed materials and the shape of the contact surface hold the portions of the unified object together. The need to bind together separately formed pieces as a post processing step is avoided.

[0004] In a first aspect, a system is provided for interlock between materials in multiple material additive manufacturing. An additive manufacturing printer is configured to form an object from two or more different materials. A controller is configured to control the additive manufacturing printer to form the object. The controller is configured to control based on a print model defining two or more different portions of the object for the two or more different materials. The print model represents a contact surface of a first one of the two or more different portions and corresponding first one of the different materials with a second one of the two or more different portions and corresponding second one of the different materials. The first portion is to be printed on the second portion forming the object as a single part from the different materials. The contact surface forms an interlock boundary shaped in three dimensions to structurally prevent separation of the first portion from the second portion along at least one dimension.

[0005] In a second aspect, a method is provided for joining between different materials by a three-dimensional printer. The three-dimensional printer prints first layers of a first part of an object with first material. The three-dimensional printer prints second layers of a second part of the object with second material. At least some of the second layers bond with at least some of the first layers to form the object. An interface between the first and second parts of the object is formed from the first and second layers. The interface prevents separation of the first part from the second part with a geometric structure.

[0006] In a third aspect, a body is formed by three-dimensional printing. The body has a first part formed by three-dimensional printing a first material and a second part formed by three-dimensional printing a second material different than the first material. The first and second parts are printed together to form the body as a unified object. The first part has a three- dimensional surface of contact with the second part. The three-dimensional surface is formed by the three-dimensional printing of the first and second parts. The three-dimensional surface interlocks the first part to the second part based on a geometric shape.

[0007] The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. Further aspects and advantages of the invention are discussed below in conjunction with the preferred embodiments and may be later claimed independently or in combination.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The components and the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views.

[0009] Figure 1 is a block diagram of one embodiment of a system for interlock between materials in multiple material additive manufacturing;

[0010] Figure 2 illustrates an example printed object with a graphic for an interlocking contact surface;

[0011] Figure 3 illustrates the example printed object without the graphic;

[0012] Figure 4 is a cut away view illustrating another example of the printed object with a different interlocking contact surface;

[0013] Figure 5 illustrates yet another example of the printed object with yet another interlocking contact surface;

[0014] Figure 6 is a view of the example of Figure 5 with one portion of the object shown transparent and without a graphic for the contact surface; and

[0015] Figure 7 is a flow chart diagram of one embodiment of a method for joining between different materials by a three-dimensional printer.

DETAILED DESCRIPTION OF THE DRAWINGS AND PRESENTLY PREFERRED EMBODIMENTS

[0016] Interlocking materials are connected for multiple material additive manufacturing. The regions of different materials in the object contact using an interlocking 3D surface. This is similar to the 2D case of puzzle pieces that create interlocking pieces of cardboard. A multiple material additive

manufacturing machines is able to create a single part without the added steps of gluing, fastening, welding, or joining as a post processing step. This reduces cost and allows for multiple materials to be combined securely in a design having unique mechanical or performance characteristics. [0017] Figure 1 shows one embodiment of a system for interlock between materials in multiple material additive manufacturing. A unified object is to be printed with different materials. To better hold the object together, the surface between the different materials has a 3D geometry to interlock a part formed from one material with a part formed from the other material.

In addition to any bonding due to printing the materials together, the 3D geometry holds the parts together to maintain integrity of the unified object. The system implements the method of Figure 7 or another method.

[0018] The system is an additive manufacturing printer 10. The additive manufacturing printer 10 includes a processor 11 , a memory 12, first and second materials 14, 15, first and second nozzles 16, 17, a platform 18, and an object model 19. The printer 10 is an additive manufacturing printer, such as a 3D printer. The printer 10 is any now known or later developed 3D printer. A reservoir, bay, or spool of plastic, metal, or other material 14, 15 connects with a deposit head (e.g., nozzles 16, 17). Multiple reservoirs, bays, and/or spools for different materials 14, 15 and corresponding nozzles 16, 17 are provided. Under the control of the processor 11 (i.e. , controller), the deposit head(s) (e.g., nozzles 16, 17) and/or support platform 18 are moved to add successive material in layers, building up the three-dimensional construction until a physical object of the object model 19 is created. A heater, heaters, or other phase transition sources make the materials malleable for extruding from the nozzles 16, 17. Any additive manufacturing system may be used.

[0019] The 3D printer 10 is a multi-material 3D printer. In one

embodiment, the printer 10 is a fused deposition machine. The controller causes selection and deposit of different materials and/or combinations of materials to print the single object. Different parts of the object are printed with different materials, providing material property variation in the object.

The print materials are mapped to provide desired performance and/or mechanical properties for different parts of the unified object. In alternative embodiments, the printer 10 is a single material printer that allows for switching between materials. [0020] For printer, the controller (e.g., processor 11 ) of the 3D printer 10 receives a 3D printer formatted object model 19 as a CAD design and/or as instructions to print the object based on the model. The object model 19 and operating instructions from the memory 12 configure the printer 10, via the controller, to form the object from two or more different materials.

[0021] Additional, different, or fewer components may be provided. For example, a single nozzle capable of additive printing of different materials is used. As another example, the processor 11 and/or memory 12 are separate from the printer 10, such as being connected to the printer 10 by wired or wireless communications. As another example, a user interface is provided.

In yet another example, the platform 18 is not provided where the printer 10 forms the object in place or on a surface separate from the printer 10. In other examples, a display is provided for displaying the object model 19.

[0022] The processor 11 and memory 12 are part of the printer 10 or another system. Alternatively, the processor 11 and/or memory 12 are part of a workstation or computer for computer assisted design (CAD). In other embodiments, the processor 11 and/or memory 12 are a personal computer, such as desktop or laptop, a workstation, a server, a tablet, or combinations thereof.

[0023] The different materials 14, 15 have different mechanical,

performance, and/or other characteristics. For example, different elasticity, stiffness, softness, flexibility, stretchability, color, opaqueness, melting points, porosity, thermal conduction, and/or impact resistance are provided by the different materials 14, 15. The different materials alternatively and/or additionally have different performance characteristics. For example, different electrical conductivity and/or magnetic characteristics are provided by the different materials 14, 15.

[0024] The memory 12 is a graphics processing memory, video random access memory, random access memory, system memory, cache memory, hard drive, optical media, magnetic media, flash drive, buffer, database, combinations thereof, or other now known or later developed memory device for storing the object model 19. The memory 12 is part of the printer 10, a computer associated with the processor 11 , a database, another system, or a standalone device.

[0025] The memory 12 stores the object model 19. The object model 19 is a print model. The print model defines the geometrical structure for printing. The print model may be a 3D print file, such as a file used by the printer 10 to define the layers and movements to print the different layers with different materials. Alternatively, the object or print model 19 is a design model, such as a 3D mesh, CAD model, or volume model of boundaries for the different materials. The processor 11 or another processor may convert the object model 19 to a print file.

[0026] The memory 12 or other memory is alternatively or additionally a computer readable storage medium storing data representing instructions executable by the programmed processor 11 for controlling the printer 10 based on the object model 19. The instructions for implementing the processes, methods and/or techniques discussed herein are provided on non- transitory computer-readable storage media or memories, such as a cache, buffer, RAM, removable media, hard drive, or other computer readable storage media. Non-transitory computer readable storage media include various types of volatile and nonvolatile storage media. The functions, acts or tasks illustrated in the figures or described herein are executed in response to one or more sets of instructions stored in or on computer readable storage media. The functions, acts, or tasks are independent of the particular type of instructions set, storage media, processor or processing strategy and may be performed by software, hardware, integrated circuits, firmware, micro code and the like, operating alone, or in combination. Likewise, processing strategies may include multiprocessing, multitasking, parallel processing, and the like.

[0027] In one embodiment, the instructions are stored on a removable media device for reading by local or remote systems. In other embodiments, the instructions are stored in a remote location for transfer through a computer network or over telephone lines. In yet other embodiments, the instructions are stored within a given computer, CPU, GPU, or system. [0028] The processor 11 is a controller of the printer 10, so is configured by the object model 19 and/or instructions based on the object model 19 to control the additive manufacturing printer 10 (e.g., control the nozzles 16, 17 and platform 18) to form the object from multiple materials.

[0029] The processor 11 is a general processor, central processing unit, control processor, graphics processor, digital signal processor, three- dimensional rendering processor, image processor, application specific integrated circuit, field programmable gate array, digital circuit, analog circuit, combinations thereof, or other now known or later developed device for causing the printer 10 to print the object. The processor 11 is a single device or multiple devices operating in serial, parallel, or separately. The processor 11 may be a main processor of a computer, such as a laptop or desktop computer, or may be a processor for handling some tasks in a larger system, such as in the printer 10. The processor 11 is configured by instructions, firmware, hardware, and/or software to be able to perform the acts discussed herein.

[0030] The processor 11 is configured to control the printer 10 based on the object model 19. The object model 19 defines two or more different portions of the object for the two or more different materials. The definition is by geometric structure (e.g., shape) and/or by points, lines or layers for 3D printing. Any number of portions may be provided, such as one part of one material and two or more other parts of a different material.

[0031] Figure 2 shows an example of the object 20 represented by the object model 19. The object model 19 defines the different parts and corresponding contact surface or surfaces 26. The defined object 20 of Figure 2 is a cylinder. Other shapes, including more complex shapes such as a branching structure, may be used.

[0032] The object 20 is formed as a single part of multiple materials. This unified object is printed as one object with the parts 22, 24 of different materials at least partially bonded to each other. Alternatively, the materials do not bond with each other despite not being intended to move relative to each other. By printing the interlocking parts together, a unified structure is made. The different parts 22, 24 connect to form one device rather than two devices designed to move relative to each other. The different materials form different parts 22, 24, but some blending may be used.

[0033] A contact surface 26 is formed between the parts 22, 24 for the different material. One portion 22 of the object is formed from one material 14. Another portion 24 of the object is formed from another material 15.

Where the two portions 22, 24 contact, the contact surface 26 is formed. As the object 20 is additively built up, the parts 22, 24 and corresponding contact surface 26 are formed. One portion 22 is printed onto another portion 24.

This printing may additively form different segments of the different portions 22, 24 iteratively, such as printing a stripe of one material followed by a stripe of another material all in a same layer. The contact surface 26 is formed additively. Since one material may be printed over or against another material, the materials may bond to each other, even if the bond is less than of the material to itself.

[0034] Since the object 20 is a 3D object, the contact surface 26 between the portions 22, 24 is a 3D surface. In a simple approach, the 3D surface may be a flat plane. Such contact surface 26 may delaminate. To better maintain the integrity of the object 20, the contact surface 26 forms an interlock boundary. This boundary is shaped in 3D to structurally prevent separation of one portion 22 from another portion 24 along at least one dimension.

The interlock boundary is shaped to structurally prevent separation of the parts 22, 24 without deforming a geometry of one or both of the parts 22, 24. Two or more pieces of geometry cannot be separated in one or more directions without deforming the geometry.

[0035] In the example of Figure 2, the contact surface 26 includes both convex and concave portions. An alternating series of concave and convex portions are provided. By mating the two portions 22, 24 as printed with this contact surface 26, the convex and concave portions interlock the portions 22, 24 together. In 3D, the interlocking connectors are in multiple directions to avoid separation. Rather than splitting the body with a plane, a surface with alternating convex and concave pieces is used to split the body. Figure 3 shows the object 20 without the conceptual graphic of the contact surface 26. The contact surface 26 of the portions 22, 24 has the alternating concave and convex portions as the interlocking boundary.

[0036] Other contact boundaries 26 may be used for interlocking. For example, one or more protrusions are provided where the protrusion is wider spaced away from the remainder of the portion than at the remainder of the portion (see the 2D cross-section of a protrusion in the object model 19 of Figure 1 ). The protrusion extends from a part into another part, where the protrusion is narrower along at least one dimension at the part from which the protrusion extends than the protrusion within the other part. The concave and convex arrangement provides multiple such protrusions. The protrusion may have any shape, such as an inverted partial pyramid shape. Rather than a gradual curve, a shape with edges may be used. A combination of edges and curves may be used. Flaving a narrower portion of the protrusion by the reminder of the part of the same material prevents separation of the parts 22, 24 from each other by pulling away. Protrusions with other shapes may prevent separation in other directions and/or rotation. One protrusion or multiple protrusions may be used. The shape is provided in cross-section.

The same or different shape is provided in an orthogonal cross-section (i.e. , 3D surface).

[0037] The interlock boundary may prevent movement in any one or more of six degrees of freedom. Depending on the use of the object, greater stress or strain may be provided along some dimensions or over some rotations. As a result, the interlocking boundary may be used to prevent translation and/or rotation in desired directions, avoiding delamination or overcoming of the weaker bond of one material to another material as 3D printed.

[0038] For example, the alternating convex and concave boundary of Figures 2 and 3 prevents rotation about any of the three axes by the linearly extending (e.g., along the x dimension) protrusions, prevents vertical (e.g., along the z dimension) translation, and prevents left/right (e.g., along the y dimension) translation. If the parts 22, 24 were frictionless, it would be possible to separate the parts 22, 24 by translation in the X direction. The friction due to the greater surface area formed by the contact surface 26 as well as the bond between materials as printed may prevent translation in the X direction.

[0039] Figure 4 shows another embodiment using alternating concave and convex shape for the interlock boundary of the contact surface 26. Some of the upper portion 22 of one material is cut away to show the contact surface 26. The concave and convex curves are formed as circular structures for interlocking. This circular arrangement with wider parts spaced away from the reminder of the portion of the same material may prevent translation in all three directions. The portions 22, 24 are locked together at the moment of creation and cannot be separated without deforming the geometry. Rotation in two directions is prevented, but rotation about the z axis is not prevented by the shape of the contact surface 26. The friction due to the greater surface area formed by the contact surface 26 as well as the bond between materials as printed may prevent rotation about the z-axis.

[0040] It may be desired to fix all six degrees of freedom (i.e. , three rotation and thee translation). A more“freeform” interlocking surface 26 may be defined to separate the parts 22, 24, providing the desired mechanical properties. Figures 5 and 6 show an example. Figure 5 shows the contact surface 26 with a graphic for both parts 22, 24. Figure 6 shows one part 22 as transparent to better view the contact surface 26 of the other part 24. The alternating concave and convex pattern or shape is used. Instead of forming straight lines along the x dimension as in Figure 2 or the circles of Figure 4, a wave is provided in the x and/or y dimensions. This contact surface 26 interlocks in a way that uses the geometry to resist movement in six degrees of freedom.

[0041] Other contact surfaces 26 may be used. An alternating pattern of other structures, a single structure (e.g., protrusion and corresponding indentation), multiple separate structures having the same or different shape, or other 3D surface geometries may be used. The 3D surface geometry interlocks the parts 22, 24 together. The contact surface 26 of both parts 22, 24 have a same shape or mate. A gap may be provided between the parts 22, 24, such as providing a tunnel, hole, or void when mated, while still interlocking. [0042] Figure 7 shows a method for joining between different materials by a three-dimensional printer. An object model 19 has an interface between parts of a same object to be formed from different materials. The interface is between the materials. Rather than relying on bonding of different materials to each other, the interface is shaped in 3D to limit translation and/or rotation of one part of the object relative to another. An interlocking interface is formed so that deformation of the part is required to translate and/or rotate about one or more degrees of freedom.

[0043] The method is performed by the printer 10 of Figure 1 or another 3D printer. For example, a multi-material 3D printer is used to print a whole object formed as one piece (e.g., single and/or unified object) with different parts being of different materials. The object model 19 is used to control the printing of both materials by the printer 10 in acts 70 and 72 in a way that forms the interlock interface of act 74.

[0044] The method is performed in the order shown or a different order. Acts 70 and 72 are performed in any order. Since additive 3D printing is used, acts 70 and 72 may be interleaved across a given layer (i.e. , printing with one material, then another, then the one material, then the other, . . . ). Simultaneous printing 70, 72 by two different nozzles of the different materials may be used. Act 74 is performed as part of printing in acts 70 and 72, so is performed simultaneously with act 70 and/or act 72, at least at times.

[0045] Additional, different, or fewer acts may be provided. For example, an act for moving the nozzles and/or platform is provided. As another example, acts for finishing, such as sanding or removal from the platform, are provided. In yet another example, acts for initiating the printing, such as loading the materials and heating the nozzles, are provided.

[0046] In acts 70 and 72, the 3D print model is 3D printed. The object model may be reformatted or compiled into instructions for printing.

Alternatively, the model includes the compiled instructions. In response to the model, the 3D printer prints out plastic, metal, paper, or other material for the different parts. [0047] In act 70, the 3D printer prints one or more layers of an object. The object is divided into at least two parts, but three or more parts may be provided. Each is part of a whole object formed as a unified piece from the different parts.

[0048] For a given part of the same material, additive printing with that material is used. The nozzle or part is moved in a pattern to deposit extruded material. Layers of material are built up to form the part. The part has any shape.

[0049] In act 72, the 3D printer prints one or more layers of the object with a different material. A different part is formed by printing with a different material. For the part, the same material is used in the additive printing. The same or different nozzle extrudes the different material as the nozzle or part is moved.

[0050] The different materials used by the 3D printer may have different characteristics, such as different mechanical and/or performance

characteristics. The different materials are used for different locations in the printed object. The resulting unified object has different parts with different properties.

[0051] Due to orientation of the object for printing, a given layer may be formed from multiple materials. Similarly, due to the 3D interlock at the interface of different materials or respective parts, a given layer may be formed from multiple materials. The printing of acts 70 and 72 for the different materials is performed sequentially to form a layer from the different materials. Interleaved printing may be used.

[0052] As material for one part is deposited on or against material for another part, the materials may bond. This bond may be stronger or weaker than the bond formed by additive printing of the same material against itself.

[0053] In act 74, the printing of layers in acts 70 and 72 forms the interface between the parts of the object. The materials are extruded against each other, forming the interface. While the materials may bond, the interface is shaped to prevent separation of one part from another part. The interface has a geometric structure to prevent separation. For example, the interface includes convex and concave portions formed in each of the parts, which are mated together by the convex and concave portions. Any protrusion from one part into the other part where the protrusion is wider further away from the one part may prevent separation. Other 3D interface structures to interlock the parts together may be used.

[0054] The geometric structure prevents separation along any degree of freedom (i.e., along one or more of six degrees of freedom). Bonding of the materials to each other may prevent separation along all the degrees of freedom. The geometric structure provides additional prevention or resistance to separation of the parts. The geometric structure may be used to prevent rotation about any or all of three dimensions and/or to prevent translation about any or all the three dimensions.

[0055] The 3D printing forms a body. The body has different parts formed from different materials. For example, a multiple material printer prints the different parts together in the one body. The body is formed as a unified object with the parts contacting each other. The unified body is one or a single piece from multiple materials, so the parts are not formed to move relative to each other.

[0056] The body includes a 3D surface of contact of the parts with each other. The 3D surface is formed by the 3D printing of the parts. The 3D surface is shaped to interlock one part with another part. The geometric shape of the 3D surface interlocks the parts together. Any 3D shape for interlocking may be used, such as one or more protrusion from one part into the other part, from the other part into the one part, or both. One or more, but not necessarily all, of the protrusions are narrower at the base than within the other part into which the protrusion protrudes.

[0057] Figures 2-6 show example bodies as a cylinder. Other body shapes may be provided. The bodies include respective 3D surfaces for interlocking, such as using alternating convex and concave structures for the parts as represented in Figures 2-6.

[0058] While the invention has been described above by reference to various embodiments, it should be understood that many changes and modifications can be made without departing from the scope of the invention. It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, that are intended to define the spirit and scope of this invention.

Claims

I (WE) CLAIM:

1. A system for interlock between materials (14, 15) in multiple material additive manufacturing, the system comprising:

an additive manufacturing printer (10) configured to form an object (20) from two or more different materials (14, 15); and

a controller (11 ) configured to control the additive manufacturing printer (10) to form the object (20), the controller (11 ) configured to control based on a print model (19) defining two or more different portions (22, 24) of the object (20) for the two or more different materials (14, 15), the print model (19) representing a contract surface (26) of a first one of the two or more different portions (22, 24) and corresponding first one of the different materials (14, 15) with a second one of the two or more different portions (22, 24) and

corresponding second one of the different materials (14, 15), the first portion (22) to be printed on the second portion (24) forming (74) the object (20) as a single part from the different materials (14, 15), the contract surface (26) forming (74) an interlock boundary shaped in three dimensions to structurally prevent separation of the first portion (22) from the second portion (24) along at least one dimension.

2. The system of claim 1 further comprising a memory (12), the memory (12) storing the print model (19).

3. The system of claim 1 wherein the additive manufacturing printer (10) comprises a fused deposition machine having two or more nozzles (16, 17), one of the nozzles (16, 17) being for one of the different materials (14, 15) and another of the nozzles (16, 17) being for another one of the different materials (14, 15).

4. The system of claim 1 wherein the different materials (14, 15) have different mechanical characteristics.

5. The system of claim 1 wherein the different materials (14, 15) have different performance characteristics.

6. The system of claim 1 wherein the interlock boundary is shaped to structurally prevent separation without deforming (74) a geometry of the first or second portion (24).

7. The system of claim 1 wherein the interlock boundary comprises convex and concave portions (22, 24).

8. The system of claim 7 wherein the interlock boundary comprises an alternating pattern of the convex and concave portions (22, 24).

9. The system of claim 1 wherein the interlock boundary is shaped to allow rotation and prevent translation in the three dimensions.

10. The system of claim 1 wherein the interlock boundary is shaped to prevent relative movement of the first portion (22) to the second portion (24) in six degrees of freedom.

11. The system of claim 1 wherein the interlock boundary includes a first extension from the first portion (22) that becomes wider further from the first portion (22).

12. A method for joining between different materials (14, 15) by a three- dimensional printer (10), the method comprising:

printing (70), by the three-dimensional printer (10), first layers of a first part (22) of an object (20) with first material (14);

printing (72), by the three-dimensional printer (10), second layers of a second part (24) of the object (20) with second material (15), at least some of the second layers bonding with at least some of the first layers to form the object (20); and

forming (74) from the first and second layers an interface (26) between the first and second parts (22, 24) of the object (20), the interface (26) preventing separation of the first part (22) from the second part (24) with a geometric structure.

13. The method of claim 12 wherein forming (74) comprises forming (74) the first and second parts (22, 24) to have mated convex and concave portions (22, 24).

14. The method of claim 12 wherein forming (74) comprises forming (74) the first part (22) to have a protrusion into the second part (24), the protrusion being wider further away from the first part (22).

15. The method of claim 12 wherein forming (74) comprises forming (74) the interface (26) to prevent separation along three dimensions.

16. The method of claim 12 wherein forming (74) comprises forming (74) the interface (26) to prevent rotation about any of three dimensions.

17. A body formed by three-dimensional printing, the body comprising: a first part (22) formed by three-dimensional printing a first material

(14);

a second part (24) formed by three-dimensional printing a second material (15) different than the first material (14), the first and second parts (22, 24) printed together to form the body as a unified object (20);

a three-dimensional surface (26) of contact of the first part (22) with the second part (24), the three-dimensional surface (26) formed by the three- dimensional printing of the first and second parts (22, 24), the three- dimensional surface (26) interlocks the first part (22) to the second part (24) based on a geometric shape.

18. The body of claim 17 wherein the three-dimensional surface (26) includes a protrusion from the first part (22) into the second part (24), the protrusion being narrower at the first part (22) than within the second part (24).

19. The body of claim 17 wherein the first and second parts (22, 24) are formed by a multiple material printer (10).

20. The body of claim 17 wherein the three-dimensional surface (26) interlocks with alternating convex and concave structures for the first and second parts (22, 24).